In recent years with the proliferation of semiconductor fabrication techniques, a number of microelectromechanical (MEMS) structures have been developed in order to reduce the size and weight of a variety of mechanical and/or electromechanical systems. For instance, some gimbal systems have been replaced by gyroscopes that include one or more MEMS devices. An example of these gyroscopes is described in U.S. Pat. No. 5,650,568 to Paul Greiff et al., the contents of which are incorporated herein by reference. The Greiff '568 patent describes a gimballed vibrating wheel gyroscope for detecting rotational rates in inertial space. As described, the Greiff gyroscope includes a number of lightweight, miniaturized MEMS devices and, as such, has been used in place of the traditional larger and heavier gimbals.
An important advantage in the use of MEMS devices for mechanical and electromechanical systems is the reduction of size and weight that can be achieved over the conventional mechanical systems. However, many mechanical and electromechanical systems, such as the gimballed systems, have many moving parts that must be accurately fabricated in order to operate properly with the requisite accuracy and precision. Thus, the ability to replace conventional mechanical and electromechanical parts with MEMS devices fabricated by semiconductor techniques is limited by the precision that can be achieved with the semiconductor fabrication techniques.
Since the size of most MEMS devices is limited, an array of MEMS elements must oftentimes be used in order to cover a broader area. For example, an inertial sensor can include a number of angular sensor elements, each of which is a separate MEMS device. Although it would be preferred that each element of a MEMS array were identical, conventional arrays of MEMS elements have significant interelement variations in size and shape. In order to appropriately process the signals generated by or provided to the MEMS elements, separate conditioning electronics must oftentimes be used for each MEMS element in order to individually process the signals. As will be apparent, the customized electronics that must be utilized in conjunction with each MEMS element significantly increases the cost and complexity of the resulting array.
Although various semiconductor fabrication techniques have been utilized to manufacture MEMS devices, these fabrication procedures are typically unable to provide the precision required by modern applications, such as the inertial sensing application described above. For purposes of further explanation, however, the fabrication techniques described by the Greiff '568 patent will be discussed below. In this regard, FIGS. 1A-1D illustrate a typical method for manufacturing MEMS devices with a conventional MEMS fabrication technique. The process illustrated in these figures is commonly known as a Dissolved Wafer Process (DWP) and is described in more detail in the Greiff '568 patent.
In particular, with reference to FIG. 1A, a silicon substrate 10 and a support substrate 12, typically formed of an insulating material, such as PYREX.RTM. glass, are shown. In a typical MEMS device, the silicon substrate is etched to form the mechanical and/or electromechanical members of the device. The mechanical and/or electromechanical members are generally supported above the support substrate such that the mechanical and/or electromechanical members are free to move.
As illustrated in FIG. 1A, support members 14 are initially etched from the inner surface of the silicon substrate. These support members are commonly known as mesas and are formed by etching, such as with potassium hydroxide (KOH), those portions of the inner surface of the silicon substrate that are exposed through an appropriately patterned layer of photoresist 16. Preferably, the etching is continued until mesas 14 of a sufficient height have been formed.
With reference to FIG. 1B, the etched inner surface 18 of the silicon substrate is thereafter doped, such as with boron, to provide a doped region 20 having a high doping concentration, such as 1 or 2.times.10.sup.20 atom/cm.sup.2, and a predetermined depth. The resulting silicon substrate 10 therefore has both a doped region 20 and an undoped sacrificial region 22. Referring to FIG. 1C, trenches are then formed, such as by a reactive ion etching (RIE), that extend through the doped region 20 of the silicon substrate 10. These trenches eventually define the size and shape of the mechanical and/or electromechanical members of the MEMS device.
As shown in FIGS. 1A-1C, the support substrate 12 is also initially etched and metal electrodes 26 and conductive traces (not shown), are formed on the inner surface of the support substrate. These electrodes and conductive traces will subsequently provide electrical connections to the various mechanical and/or electromechanical members of the MEMS device.
Once the support substrate 12 is processed to form the electrodes and conductive traces, the silicon substrate 10 and the support substrate 12 are bonded together. With reference to FIG. 1D, the silicon and support substrates are bonded together at contact surfaces 28 on the mesas 14, such as by an anodic bond. As a final step, the undoped sacrificial region 22 of the silicon substrate is typically etched with a doping sensitive etch, such as ethylenediamine pyrocatechol (EDP). EDP is a wet etchant that selectively etches the undoped silicon. As such, only the doped region that comprises the mechanical and/or electromechanical member of the resulting MEMS device remains following the etching procedure. The mesas that extend outwardly from the silicon substrate therefore support the mechanical and/or electromechanical members above the support substrate such that the members have freedom of movement. Further, the electrodes formed by the support substrate provide an electrical connection to the mechanical and/or electromechanical members through the contact of the mesas with the electrodes.
While EDP selectively etches the undoped silicon relative to the doped silicon to form the mechanical and/or electromechanical members, the mechanical and/or electromechanical members cannot be precisely defined to within the tolerances demanded by certain applications, such as to within a fraction of a micron for inertial sensing applications, due to non-uniform effects such as slight variations in the doping concentration of the silicon. As such, the mechanical and/or electromechanical members cannot be repeatedly fabricated to within the desired tolerances and the corresponding performance of the resulting MEMS devices will therefore be somewhat different. As described above, an array of MEMS devices would therefore typically require separate electronics for each element since the performance of each element would likely be different than the performance of other elements. As known to those skilled in the art, variations in the doping concentration also has other disadvantageous effects, such as curling of the resulting mechanical and/or electromechanical member. As a result the high doping levels required for selective etching with EDP, crystal structure imperfections can be created and the resonant Q and Young's modulus of the resulting mechanical and/or electromechanical member can vary. Finally, EDP is generally considered toxic and therefore raises a number of environmental concerns and requires specialized and costly handling and disposal procedures.
While a variety of MEMS devices have been developed for reducing the size and weight of the resulting mechanical and/or electromechanical system, conventional MEMS devices and their respective fabrication procedures have not been entirely satisfactory for all applications. In particular, it remains a difficult challenge to fabricate a plurality of MEMS devices having a precisely repeatable size and shape in order to ensure consistent operation, specifically in the context of an array of MEMS devices. In addition, the dissolved wafer processing technique typically utilized to fabricate MEMS structures requires the use of EDP which further increases the complexity of the fabrication process as a result of the specialized handling and disposal procedures that must be followed in order to properly use EDP.